Coating for pistons



Patented Oct. 9, 1934 UNITED STATES PATENT OFFICE COATING FOR PISTONS No Drawing. Application August 24, 1932, Serial No. 630,245

Claims.

' The invention relates to a method of applying a permanent and wear-resisting coating on pistons for internal combustion engines or the like.

Pistons made from aluminum alloys or other light metals and alloys are now in use in large numbers. Unless properly andcarefully designed and lubricated these light alloy pistons have a tendency to wear slightly more than pistons of the harder and heavier metals, but by reason of m the many advantages attendant on their use,

structural and design methods have been developed which, to some extent, lessen or compensate for the tendency to wear.

It is the purpose of the present invention to provide a method whereby the wear-resisting and bearing properties of light alloy pistons may be greatly improved.

I Attempts have heretofore been made to apply coatings of tin to aluminum alloy pistons but a coating sufiiciently adherent to withstand for a satisfactory period of time the conditions existing in a cylinder of an internal combustion engine in operation was not obtained and efforts to use tincoated aluminum base alloy pistons have met but with temporary success.

The present invention has for its purpose the coating of an aluminum alloy piston with tin by a method whereby a permanent bond is established between the metal of the piston and the coating metal. This I have accomplished by applying to the piston an intermediate coating of a harder metal, such as nickel,and then superimposing a coating of tin. over the harder metal. I have found that a coating of tin applied in this way is quite permanent, having no tendency to scale off or scuff.

The double coating is preferably applied electrolytically and I have found the following to be {w a satisfactory course of procedure, although other methods of coating may be employed. I first clean the machined. piston in a mild alkaline cleaner made up of about equal parts of sodium carbonate and trisodium phosphate of a concentration of approximately one to three ounces total in a gallon of water. This procedure removes any possible traces of oil or any type of organic solvent. This bath will function best at about 200 Fahrenheit and the cleaning operation should be followed by a cold water rinse.

The piston surface may now be roughened by dipping in a solution of the following composition:

Nitric acid (HNOa) (Sp. Gr. 1.42) 3 parts 65 Hydrofiuoric acid (HF) (48-52%) 1 part be varied depending on the material to be plated each day.

Nickel sulfate (NiSO4 71120) 19 oz. per gal. water Magnesium sulfate (MgSO4 71-120) l0 oz.per gal. water Ammonium chloride (NH4C1) 2 oz. per gal. water Boric acid (HaBOa) 202. per gal. water The current density employed should be in the order of 15 amperes per square foot but this may and the plating speed. At 15 amperes per square 5 foot a thicknessof slightly under a thousandth (.001) of an inch of nickel per hour is obtained.

For the best results the nickel coating should be 0.0003 inch or more in thickness. I believe that the function of the nickel or other hard metal, such as, for instance, chromium or cobalt, is to anchor itself into the minute pits etched out by the dipping solution and thus serve as a solid foundation to whichthe coating of tin adheres much more readily than to the aluminum itself.

A final coating of tin is superimposed upon the nickel by any standard method such as by electrodeposition from a bath containing 2 to 3 per cent stannous chloride, 5.5 to 6.5 per cent ammonium oxalate and 0.3 to 0.4 per cent oxalic acidusing a current density of about 3.6 amperes per square foot. I have found that a coating of about 0.0004 inch of tin over nickel gives highly satisfactory results. The tin coating may be made thicker or thinner than this figure, depending on service requirements or the demands of the manufacturer or customer.

A number of tests were run in a commercial eight cylinder engine for the purpose of comparing different coating combinations. All tests' were run for about two months in a motor which ran intermittently, starting cold at least twice A piston plated with 0.0004 inch of tin directly on aluminum was scuffed on the maximum thrust side and the plated surface was non-uniform.

Unplated aluminum pistons simultaneously running in the same engine were slightly scuffed.

In these last three pistons there were no signs of smiling, spotting, or irregularity in the bearing surface.

I have determined that this double coating on an aluminum or aluminum-base alloy piston, consisting of a coating of tin superimposed upon a harder metal over the aluminum, forms the best possible combination. It combines to some extent the advantages of a hard metal and a soft metal, exhibiting simultaneously the properties of adhesiveness, tenacity, and good bearing properties. In its broadest aspect my invention contemplates a combination coating of a'soft metal and a hard metal so chosen that there will be present at the same time a good bearing surface and an adherent coating but I specifically prefer to use a combination coating composed of an intermediate nickel layer and an outer tin layer.

Having thus described my invention, I claim:

1. A method of improving the bearing surface of alight alloy piston which consists in applying upon said piston a coating of a metal selected from the group comprising nickel, chromium and cobalt, and superimposing upon said coating an additional outer coating of tin.

2. A method of applying upon an aluminum base alloy piston a coating of tin which consists in applying a coating of nickel intermediate the surface of said piston and the tin coating.

3. A method of producing an aluminum base alloy piston characterized by a long service life and good bearing properties which consists in first electroplating upon said piston a coating of nickel more than 0.0003 inch in thickness, then electroplating upon said nickel a coating of tin.

4. A method of producing an aluminum base alloy piston characterized by a long service life and good bearing properties which consists in first machining the piston, then cleaning the piston surface with a mild alkaline cleaner, then roughening said cleaned surface by dipping the piston in a corrosive'solution, then electroplating upon said roughened surface a coating of a metal selected from the group comprising nickel, chromium and cobalt, and then electroplating upon said coating a coating of tin.

5. A method of producing an aluminum base alloy piston characterized by a long service life and good bearing properties which consists in first applying upon said piston a coating of a metal selected from the group comprising nickel, chromium and cobalt, then electroplating upon said-coating a coating of tin.

HAROLD K. WORK. 

